This study will probe the role of gene interactions in mouse neural tube development. Formation of the neural tube is one of the first, most critical events in mammalian embryogenesis, and defects in the process (including genetic ones) are among the most common human birth defects. The study will make use of three newly induced mutations in the mouse, isolated on the basis of their interactions with loci known to affect neural tube development. These new mutants will be characterized genetically and developmentally, determining their morphological effects in embryos and the modifications they impose on other neural tube mutations already described. Studies will include linkage and mapping, histological analysis of embryos, construction of double mutants and determination of lethal phases. The new mutants were isolated for their interactions with the Splotch locus, which causes failure of neural tube closure in homozygous condition, and Fused, which produces duplications of the neural tube in homozygotes. The study of new interacting loci will help define the set of genes whose coordinate action is required for normal neural tube formation. Since many of the genes that direct neural tube development are likely to mediate cell interactions, it will be essential to establish if a mutant defect is intrinsic to cells of the neural plate, or whether it results from an abnormal cell interaction of extrinsic origin. In order to address this question, we will construct genetic mosaics by aggregating mutant and wild-type embryos. The cells' genotypes will be scored with a new molecular marker, a gene fusion placing bacterial Beta-galactosidase under the control of a eukaryotic promoter, which will be introduced into the germ line of a normal mouse strain. Thus, it will be possible to score the distribution of mutant vs. wild-type cells, using antibody staining of sectioned embryos. In this manner, a correlation can be made as to which cells must carry a particular mutation in order to produce the mutant phenotype, defining the cellular site of action of a given mutation. This technique will be used to map the various genetic and tissue contributions to neural tube development, as well as to trace the cell lineage of the neural tube. In the long run, studies of this sort will define the set of genes required for neural tube formation and lay the foundations for molecular studies of this process.